This invention relates to a semiconductor module for power (hereinafter referred to as "a power semiconductor module", when applicable) which is formed by assembling power semiconductor elements such as a plurality of insulated gate bipolar transistors (hereinafter referred to as "IGBTs", when applicable) forming a bridge circuit employed for a power inverter device or the like in a common casing together with its control circuit.
A conventional power semiconductor module of this type is designed as follows: For the purpose of reduction both in size and in manufacturing cost, the semiconductor elements are not accommodated in the respective packages, and instead the semiconductor elements which are provided in the form of chips are assembled as they are. Furthermore, in order to provide a so-called "intelligent module" with them which is convenient in practical use, they are set in a common casing together with a control circuit including a semiconductor element drive circuit, and then resin is injected into the casing. Hence, the structure of the power semiconductor module thus formed is rigid. FIGS. 7 and 8 are sectional views showing examples of the structure of the above-described conventional semiconductor module.
FIG. 7 shows two semiconductor elements 1 and two element control circuits 2 which correspond to one phase of a three-phase bridge circuit. The left half of FIG. 7 is for a description of the upper arm side which has the positive input terminal P of the bridge circuit, and the right half is for a description of the lower arm side having the negative input terminal N. The U-phase output terminal U is shown at the middle of FIG. 7. The bottom of the module is made of a metal substrate 4 of copper on which a heat-sink (not shown) is mounted. Onto the substrate 4, a ceramic board 5 is soldered through its metallized layer 5a.
A plurality of connecting conductors 6, which are arranged in a predetermined pattern, are supported on the upper surface of the ceramic board 5. The semiconductor elements 1 are each soldered, in the form of a chip, to the connecting conductors 6. The semiconductor elements 1 are connected to the connecting conductors 6 adjacent thereto with bonding wires as shown in FIG. 7, and terminal plates 6a of copper are fixedly mounted on the upper surfaces of the connecting conductors 6, for instance, by soldering in such a manner that they are extended vertical, thus being employed as the input terminals P and N and the output terminal U.
Each of the element control circuits 2 is an integrated circuit device which includes a drive circuit and a protective circuit for the respective semiconductor element 1, and it is mounted on a wiring substrate 7, which is a so-called "printed circuit board", with its leads soldered thereto through wiring conductors 7a of copper. For each of the semiconductor element 1, a plurality of bar-shaped copper supports 6b are extended upwardly from the connecting conductor 6 which is bonded to the element 1 in the rear region. The upper portions of those supports are soldered to the wiring conductors 7a of the wiring substrate 7, so that the element control circuit 2 is connected to the semiconductor element 1, and the wiring substrate 7 is supported on the bottom of the module. A plastic casing 8 is in the form of a rectangular hollow quadrangular prism. The lower edge of the plastic casing 8 is engaged with the periphery of the metal substrate 4, and the lower ends of a plurality of terminal bars 8b secured to the upper edge portion of the plastic casing 8 are suitably connected to the wiring conductors 7a of the wiring substrate 7, thus being employed as external connecting terminals for control signals. Under this condition, epoxy resin is injected into the casing 8 and then solidified, so that all components are provided as one unit. Hence, the resultant module is high in rigidity.
FIG. 8 shows an example of the structure of a module suitable for small electric power in the same manner as FIG. 7. In FIG. 8, parts corresponding functionally to those already described with reference to FIG. 7 are therefore designated by the same reference numerals or characters. Two semiconductor elements 1, which are each in the form of a chip, are fixedly mounted as they are by soldering the wiring conductors 7a of a wiring substrate 7 which is fixedly mounted on the upper surface of a metal substrate 4, for instance, by bonding. And two integrated circuit devices for two element control circuits 2 are also mounted on the right and left sides of the aforementioned wiring substrate 7.
The semiconductors, which are each in the form of a chip, are suitably connected to the wiring conductors 7a with bonding wires 9 as shown in FIG. 8. A positive input terminal P, a negative input terminal N, and an output terminal U are extended from the middle region where the element control circuits 2 are mounted, and control terminals Tc are extended from the right and left regions where the element control circuits 2 are mounted. Similarly as in the module shown in FIG. 7, the metal substrate 4 is fitted, as a bottom, in the casing 8, and resin 8c is injected into the casing 8 thus bottomed, and solidified, so that all elements are provided as one unit. That is, the resultant module thus formed is relatively flat, being smaller in height than the one shown in FIG. 8.
In the above-described prior art to FIG. 7, the ceramic board 5 and the wiring substrate 7, which bear the semiconductor elements 1 and the element control circuits 2, are arranged one above another, thus forming a so-called "two-storied structure". Hence, the resultant module is relatively small in its occupied area; however, it is disadvantageous in that its assembling work takes time and labor, and accordingly it is high in manufacturing cost. The reason why the assembling work takes time and labor so much resides in that the ceramic board 5 and the wiring substrates arranged one above another are coupled to each other through the connecting supports 6b, and that at least three to five supporting supports 6b are required for each semiconductor element 1, and, during the assembling work, the end portions of those supporting supports 6b are accurately positioned at the through-holes formed in the wiring board 7 and fitted in them (the through-holes). In addition, since a number of connecting supports 6b must be erected on the ceramic board 5, the latter 5 needs an additional area for the erection. Therefore, although the module employs the two-storied structure, it is impossible to sufficiently reduce the area occupied by the module.
The module shown in FIG. 8 is free from the above-described difficulties accompanying the one shown in FIG. 7; however it is still disadvantageous in that it is not suitable for high power because the wiring substrate 7 is considerably high in thermal resistance; that is, the semiconductor element 1 is low in thermal radiation efficiency. Since the wiring substrate 7 has no through-holes, the module is not suitable for the installation of the integrated circuit devices for the element control circuits 2.